You're supposed to get to the end of the 18th puzzle (I did w/o a problem), and then the computer tries to kill you. You escape, and then it apologizes. You go through the service hallways and eventually kill it. Then there is a funny song called "Still alive" that plays for the credits, I suggest you watch it on youtube if you have 3 minutes to burn (search for portal + still alive video). It's sung by the opera singer/voice actor who plays the combine overwatch as well. I think it's a waste considering they synthesize her voice, but that is neither here nor there.

The game was made by interns @ Valve, and Valve is now using the Aperture science story line in the next HL2 episode, with the introduction of the portal gun. If you explored Portal, there was a room where the company execs were making a presentation on how they have to beat Black Mesa with better technology etc. There are also plenty of jokes about the piece of cake you never receive, hence the name of the steam download.